[Paleontology • 2017] Daspletosaurus horneri • A New Tyrannosaur with Evidence for Anagenesis and Crocodile-like Facial Sensory System

A new species of tyrannosaurid from the upper Two Medicine Formation of Montana supports the presence of a Laramidian anagenetic (ancestor-descendant) lineage of Late Cretaceous tyrannosaurids. In concert with other anagenetic lineages of dinosaurs from the same time and place, this suggests that anagenesis could have been a widespread mechanism generating species diversity amongst dinosaurs, and perhaps beyond. We studied the excellent fossil record of the tyrannosaurid to test that hypothesis. Phylogenetic analysis places this new taxon as the sister species to Daspletosaurus torosus. However, given their close phylogenetic relationship, geographic proximity, and temporal succession, where D. torosus (~76.7–75.2 Ma) precedes the younger new species (~75.1–74.4 Ma), we argue that the two forms most likely represent a single anagenetic lineage. Daspletosaurus was an important apex predator in the late Campanian dinosaur faunas of Laramidia; its absence from later units indicates it was extinct before Tyrannosaurus rex dispersed into Laramidia from Asia. In addition to its evolutionary implications, the texture of the facial bones of the new taxon, and other derived tyrannosauroids, indicates a scaly integument with high tactile sensitivity. Most significantly, the lower jaw shows evidence for neurovasculature that is also seen in birds.

Figure 3: The growth series of Daspletosaurus horneri sp. nov., based on parsimony analysis.
Unambiguously optimized derived phylogenetic characters were recovered as synontomorphies at two of the five growth stages, which are labeled at the corresponding numbers. Scale bar equals 10 cm. Abbreviations: AMNH FARB, American Museum of Natural History, Fossil Amphibians, Reptiles, and Birds; MOR, Museum of the Rockies.

Theropoda Marsh, 1881

Tyrannosaurinae Matthew and Brown, 1922 (sensu Sereno et al., 2005)

Daspletosaurus Russell, 1970

Daspletosaurus. All species more closely related to Daspletosaurus torosus than to Tyrannosaurus rex.

Daspletosaurus horneri sp. nov.

Etymology: Horneri, Latinized form of Horner, in honor of Jack Horner, in recognition of his successful field program in the Two Medicine Formation that has recovered many new species of dinosaurs that are critical for our understanding of the palaeobiology of dinosaurs in Laramidia, support in the preparation and curation of these specimens, and to acknowledge that his mentoring efforts have launched many professional scientific careers.

Figure 4: The craniofacial epidermis of Daspletosaurus horneri sp. nov., based on comparison with its closest living relatives, crocodylians and birds. Figure 4 Bone texture indicates large zones of large, flat scales and subordinate regions of armor-like skin and cornified epidermis; integumentary sense organs occur on the flat scales that cover the densest regions of neurovascular foramina. The region outside of the crocodylian-like skin is reconstructed with small scales after fossilized skin impressions of tyrannosaurids.